8th Grade Physical Science Curriculum Unit 1: Density and Buoyancy Lesson 5. Apply: Cartesian Divers Inquiry Objectives: Students will be able to plan a scientific investigation, and collect and evaluate data. Students will learn that an object floats due to the buoyant force of the water it displaces Key Questions: What is an investigatable question? Why do the divers float, then sink? What is required for an object to function as a Cartesian diver? Time: 4 to 5 class periods Materials For each pair of students: One two liter bottle with top, Cartesian diver (plastic dropper + nut) Optional: FizzKeeper pressurizer water Resources Investigation Proposal (PDF) Can You Make a Diver From Different Objects?" (PDF) Overview Your students have learned that different objects have different densities, and that the density of an object relative to the liquid it is in will determine if it will sink or float. Cartesian Divers add a new twist. These objects float initially, but sink when put under pressure. They do this because the pressure decreases the volume of air inside the diver, meaning less water is displaced by it. This, in turn, reduces the buoyant force, causing it to sink. These divers are fun, and provoke lots of questions. Rather than answer them directly, we can use this as a chance to build valuable investigation skills. As the teacher, your role is to guide the students to define questions and hypotheses they can actually test through a practical experiment. This builds ownership of the process and the product, and results in deeper understanding than simply following the directions in a book. Assessment: Investigation Proposal (PDF) OUSD/California Science Content Standards: 1a. plan and conduct a scientific investigation to test a hypothesis. (Calif. # 8.9a) 1b. evaluate the accuracy and reproducibility of data (Calif. # 8.9b) 5c. the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid it has displaced. (Calif. #8.8c) Procedure: Step One; Exploration Distribute the divers and water bottles, and allow the students to explore. You can have your first class actually put them together and get them to work. After about 15 minutes of exploration, ask the students to record basic observations of the behavior of the diver. Ask them to likewise record three questions about the diver. In the discussion that follows, you can record their comments in a KWL format on the board.  We Know  We Want to know  We Learned (to be filled in later) Use butcher paper if you can, then you can save it to refer to their observations and questions later on. The key here is to generate as many investigatable questions as possible. You can add some of your own to the list if you wish. Investigation Challenge Make sure you and the students have a written record of all their questions. Take the questions home with you and write a master list, with all the questions from all your classes. If two questions are basically the same, just combine them. The next day give the students the list of their questions. Go through the list with them, asking this question; "Could we answer this question with an experiment?" If they say yes, ask them how they would design the experiment. Practice troubleshooting their experiments with them. What are the materials? How will you measure results? Will the experiment actually answer the question? This is potentially a very rich discussion. Next you actually ask students to choose a question from those that are investigatable, and design an experiment to try to answer it. Distribute the sheet titled Investigation Proposal. Have the students fill in their chosen question, hypothesis, materials and procedure. For the question "How will results be measured?" students are likely to need guidance. Their idea of an experiment is often just to put things together and see what happens. (The FizzKeepers are a very useful tool in this regard, because you can actually count the number of times you pump the device as a means of measuring the pressure. You will not get an absolute measure of the amount of pressure, but you can use the number of pumps as a way of comparing.) Assessment -- Peer Review The Investigation Proposal has a form for peer review built in. Once the students have written their proposals, have them share with a colleague, and review one anothers' work. You may have them do a second draft. When they are done, collect their final drafts, and take them home. Turn the best experiments into worksheets for the whole class to follow, with credit given to the student authors. You can combine a number of similar experiment proposals into one. See the sample experiment "Can You Make a Diver From Different Objects?" (PDF) In this example, students came up with a variety of objects to test as divers. This was very useful because it drew attention to the properties a diver must have if it is to function. Unit 1 Home